The preferred embodiment relates to a pneumatic actuator for setting a control armature, like a control valve, of a field device of a technical processing installation.
A known pneumatic actuator has a pneumatically impingeable working chamber connected to a pneumatic pressure supply in particular via a positioner with an integrated magnetic valve. In order to displace the control valve, the pneumatic working chamber is impinged by the positioner with a pneumatic positive or negative pressure that displaces a positioning plate into a desired control position, the plate being held at a membrane and having attached to it the control valve in a displacement-force-transmitting manner. Therein the positioner impinges on the pneumatic working chamber with a pneumatic set or control pressure signal adjusted by a control routine. In order to subsequently move the control valve back into the initial position, return forces generated for example in a second working chamber, in particular a return chamber, for example through a mechanical unit like a compression spring or through a reverse pneumatic, may act on the positioning plate.
A defined locking position is to be automatically adopted for example in case of a general operational disturbance of the technical processing installation, in particular in case of a control valve designed as a so-called safety valve. This can be achieved in that a preloaded, helical compression spring inside the return chamber pushes the positioning plate and thus the control valve into the locking or emergency position when the pneumatic working chamber is vented.
When controlling the position of the control valve a positioner computes an electrical position setpoint signal based on an actual position and on control parameters of a control center, the signal being converted by an IP-converter of the positioner into a pneumatic control pressure signal which is then output to the working chamber.
In many areas of application it can occur that a pneumatic actuator remains inactive for a prolonged period of time during which the control valve always remains in the same position, for example in a completely open or a completely closed position, and/or that the valve is subject to a critical state of wear. A positioning movement of the control valve generally occurs then when an increased control pressure is generated within a working chamber. The phenomenon of the control valve initially remaining immobile just at the beginning of the positioning movement and subsequent overshooting of the control valve beyond the desired set position resulting from the initial immobility, may occur, which is known as a stick-slip-effect. In order to overcome the initial static friction that may be particularly high for infrequently operated actuators, a correspondingly high pressure must be generated in the pneumatic working chamber.
In the case of a double action drive a pressure difference between a first and a second working chamber (main working chamber and return chamber) is to be adjusted accordingly to a large value in order to overcome the initial starting friction. As soon as the pneumatic drive displaces the control valve from its rest position, the high pressure difference forces the drive to rapidly break out from its rest position, which is difficult to control in terms of control engineering. Thus, due to the inertia of the pneumatic/mechanic components of the double action control drive, the control valve will overshoot the specified position. In many operating conditions a small change in position is desired which, owing to the stick-slip-effect, can only be achieved from the rest position by means of at least one readjustment resulting in a gradual levelling of the control valve around the specified position. A position sensor may recognize the overshooting and the positioner counteracts the latter by reducing the pressure difference at the positioning plate by reducing the pressure in the working chamber. For a double action actuator the return movement of the control valve can be achieved by subjecting the return chamber to pneumatic pressure. The control speed of the pneumatic reversal in the respective working chambers is limited by the air capacity of the positioner as well as the inertia of the system.